1. Field of the Invention
2. Description of the Related Art
The present invention relates to an outdoor termination for a high voltage cable and a manufacturing method for such an outdoor termination.
Outdoor terminations for high voltage cables typically consist of an insulator body in which the high voltage cable with its insulated cable core is accommodated. The high voltage cable is typically insulated with a polymeric material and the interior of the insulator body must be filled with an insulating filling compound to provide the necessary insulation.
FIG. 1 shows a typical construction of a inventional outdoor termination OT. It comprises an insulator body 2 having an upper cover 10 and a lower cover 11, preferably made of metal, i.e. an upper metal work 10 and a lower metal work 11. At the upper metal work 10 a conductor stalk 9 is provided with which the cable core 5.1 is connected. The lower metal work 10 is also connected to the insulator body 2 at a bottom portion thereof, for example by means of nuts and bolts 11.1.
Within the interior of the insulator body 2 the cable CA extends wherein the cable core 5.1 is surrounded by an insulation 5, which is typically made from a polymeric material.
At the lower portion of the insulator body 2 the high voltage cable CA is surrounded by an antikinking protection 7 to avoid a breakage of the cable. Also provided at the lower metal work 11 (the base plate) there is an entrance bell 8 having a connector 20.1.
Through the entrance bell 8 the filling compound 3 can be injected into the interior of the insulator body 2 such that the filling compound 3 fills at least a portion of the space between the interior walls 2.1 of the insulator body 2 and the cable insulation 5. Typically the outdoor termination is mounted in a substantially upright position such that a cavity 1 is formed at an upper portion of the insulator body 2.
Furthermore, an electric field control means 4 in the form of stress cone 4 is provided at a lower portion of the insulator body 2 around the cable insulation 5 in order to appropriately set the electric field conditions inside the insulator body 2. Typically, the insulator body 2 is made of porcelain or is a composite insulator of reinforced epoxy resin and siliconee sheds.
The critical components in the outdoor termination OT shown in FIG. 1 are of course the upper and lower metal work 10, 11 and in particular the filling compound 3 itself, with respect to the liquid/gas-tightness and with respect to possible temperature fluctuations and pressure variations.
The filling compound 3, e.g. conventionally an insulating fluid, must possess the required dielectric properties and it must be chemically inert with respect to the material of the insulator body 2, the cable insulation 5 and the material of the stress cone 4.
The insulating liquids are quite expensive. It must in particular be observed that such outdoor terminations for high voltage cables comprise rather voluminous insulator bodies 2 such that a cost-intensive filling compound 3 will drastically increase the total cost of the outdoor termination OT. Typically, the outdoor termination is between 2 and 5 m long and its interior volume is between 50 and 1000 liters.
It must furthermore be considered that the outdoor termination OT is arranged in open space and thus exposed to all kinds of environmental influences, in particular large changes in temperature and/or large stresses due to snow or wind. Temperature changes cause changes in the volumina of the filling compound 3 accompanied by pressure changes. Even when large temperature changes occur, it must be avoided under all circumstances that a leakage occurs at the bottom part of the insulator body 2. On the other hand, when there is a large drop in temperature, it must be avoided that air is sucked into the cavity 1 at the top of the insulator body 2. Generally, it is accepted that the interior of the insulator body 2 must be protected against pressures of several bar.
To ensure that neither a leakage occurs at the bottom portion of the insulator body 2 nor air is sucked into the cavity 1, the insulator body 2 must be effectively sealed and this requires components with high precision. In turn, several mounting steps are necessary when mounting the outdoor termination OT in the environment and this adds to the cost of the outdoor termination OT.
Even after installing the outdoor termination OT regular maintenance must be performed since loss of insulating liquid can substantially influence the operating characteristics of the outdoor termination.
Thus, on the one hand the substantial amount of expensive insulating liquid 3 increases the cost of the outdoor termination and on the other hand the provision of a liquid inside the insulator body 2 requires complicated mounting steps and requires regular maintenance. This applies to all high voltage cables CA, not only plastic insulated high voltage cables.
A conventional solution to reduce the cost of the outdoor termination OT is to attempt-to reduce the amount of necessary insulating fluid 3. For example, insulator bodies 2 have been suggested which have a tapered shape towards the upper portion of the insulator body 2 such that the interior volume of the insulator body 2 is decreased. In principle, this is possible because the electric field strength decreases towards the upper portion of the insulator body 2. Whilst in this manner the interior volume of the insulator body 2 can be decreased to some extent, on the other hand the manufacturing of a tapered insulator body 2 again increases the cost.
The maintenance work is essentially caused by the filling compound 3 being in a liquid state. Thus, conventionally also outdoor terminations OT have been suggested that do not require the use of an insulating fluid inside the insulator body 2. Three different possibilities have been investigated:
A first example is an outdoor termination where a rubber-elastic insulating body including an integrated stress cone is pressed into a solid insulating casing which is connected with an insulator and central conductor. Whilst the rubber-elastic insulating body provides the necessary electric insulation, this construction requires several components whose dimensions must be matched very accurately. This leads to high costs and requires several manufacturing steps also leading to an increase of the costs. Whilst the use of an insulating liquid is not necessary, on the other hand the assembly is rather rigid and thus does not easily allow some movement due to winds and short-circuits.
A second example uses a rubber-elastic or heat-shrinkable insulating sleeve. This leads to a less rigid assembly which cannot tolerate the above mentioned mechanical impacts.
A third example is suggested in xe2x80x9cElektrizitxc3xa4tswirtschaft, Jg. 99 (2000), Heft 11: Trockene Freiluftendverschlxc3xcsse mit Stxc3xctzeigenschaftenxe2x80x9d by R. Eitle and J. Kaumanns. Here, a dry rigid type outdoor termination is suggested. The solution presented here comprises a siliconee-based liquid insulating material which cross-links in the outdoor termination only after having been filled in. The insulating material is compressible. This property is achieved by xe2x80x9cmicro-spheresxe2x80x9d with which the insulating material is filled. These xe2x80x9cmicro-spheresxe2x80x9d are hollow and filled with gas. They have a size of about 100 xcexcm. The xe2x80x9cmicro-spheresxe2x80x9d serve as compensating volumina. Thus, even at extreme temperature changes no damages due to cracks or gaps are caused. Furthermore, the solid insulation material has some electrical properties.
However, even in the third example with the cross-linking siliconee-based material, there are disadvantages. Firstly, the cross-linking polymer is prepared on the basis of a siliconee elastomer and spheres are hollow and are filled with gas. The thermal conductivity of the siliconee elastomer is about 60 mW/mK and is thus only about 20% of the usual filling compound used in conventional outdoor terminations. For compensating this low thermal conductivity an increase of temperature must be avoided. Therefore, an additional heat compensating conductor having a large cross section needs to be mounted and this measure is very complicated and thus increases the costs.
A second disadvantage is the high viscosity of 17.000 mPaxc2x7s. Such a material cannot easily be inserted into the interior of the insulator body such that either large pressures are caused or long filling times are necessary. Therefore, the assembly of this type of outdoor termination is complicated (requires many different tools) and a large time is needed for the assembly.
The xe2x80x9cmicro-spheresxe2x80x9d are hollow and are filled with gas in order to provide a compensating volume, for example during temperature changes. On the other hand, the gas in the hollow cavities in an insulating medium present a higher risk even if practical measurements of the partial discharge intensities are lower than 1 pC. Furthermore, there is a risk that the gas gradually diffuses into the surrounding insulating material of siliconee elastomer and that thus the pressure in the cavities is reduced. This reduction of pressure in the cavities also causes a decrease of the ignition voltage for partial discharges. Thus, partial discharges even at the normal operating voltage can take place and can destroy the dielectric.
As explained above, the conventional outdoor terminations are either very cost-intensive and complicated to assemble since in the case of using a liquid insulating fluid measures need to be taken to avoid leakages of the fluid which in turn also causes a high maintenance effort. On the other hand, a filling compound consisting of micro-spheres and a cross-linking material on the basis of a siliconee elastomer where the micro-spheres are filled with gas avoids the leakage problems but causes additional problems of possible partial discharges and/or the need for providing additional heat conductors.
Therefore, the object of the present invention is to provide an improved outdoor termination and a method for making the same which can reduce the cost of the outdoor terminations without having the risk of partial discharges and damages to the dielectric. At the same time it is contemplated to provide an outdoor termination which allows for adjusting the electrical properties according to individual needs.
This object is solved by an outdoor termination for a high voltage cable, comprising an insulator body for receiving the high voltage cable, a filling compound provided within the insulator body and filling at least a portion of the space between the insulator body interior walls and the cable insulation wherein said filling compound consists of a mixture of a particulate material and an insulating compound.
Furthermore, the present invention also pertains to a method for making an outdoor termination for a high voltage cable comprising the following steps: preparing an insulator body for receiving the high voltage cable, inserting the filling material into the insulator body to fill at least a portion of the space between the insulator body interior walls and the cable insulation, wherein said filling material is prepared as a mixture of solid particles and an insulating compound, wherein in a first step said solid particles are inserted in said insulator body, in a second step said insulating compound is inserted into the insulator body, wherein said solid particles and said insulating compound are mixed.
In accordance with the invention, the filling material is prepared as a mixture of solid particles and an insulating compound. The term solid particles throughout this description and the claims denotes particles without gaseous inclusions, i.e. solid particles according to the present invention are not hollow particles. Because of the solid nature of these particles, the problems of partial discharges due to a possible gas leakage as in the prior art are avoided.
The solid particles according to the present invention may be made from any suitable solid insulating material. Such materials may be of an organic or inorganic nature. However, generally speaking organic particles are preferred in view of their mechanical stability. That is, inorganic materials, such as glass or ceramic materials may readily suffer from fracture and thus impact the mechanical and electrical properties of the filling compound. This is particularly true of inorganic particles in shapes other than spherical shapes.
Particularly preferred are polymeric materials, such as high, medium and/or low polyethylene particles of various degrees of cross-linking. PTFE, PVC, polypropylene and/or rubber particles can also be used.
The particles may have a variety of shapes, whereby shapes without sharp edges are preferred. Thus, generally speaking, pellet or ball shapes are preferred. However, cubic and flat shapes may also be useful. The preference of pellet- or ball-like shapes is also due to the fact that the present invention aims at filling the insulator body as uniformly as possible. That is, the aim is to provide the particles and surround the particles with the insulating compound. This aim can best be fulfilled when the particles are of a substantially ball-like shape as the interstices between ball-like particles can be filled with the insulating compound more easily than e.g. in the case of particles with a flat shape.
As far as the size of the particles is concerned, the general dimensions of the particles should be adapted to the overall design of the outdoor termination and the required insulating properties. Generally speaking, however, it is preferred to use particles with a size of 1-5 mm, whereby sizes in the range of 1-3 or 1-4 mm are particularly preferred. The term xe2x80x9csize of the particlesxe2x80x9d in the context of the present invention means that the particles satisfy the afore-mentioned requirements in all three dimensions.
In a preferred embodiment of the present invention, the particle size distribution is narrow in the sense that all particles are substantially of the same size. That is, the majority of the particles is within xc2x120%, preferably xc2x110% or even xc2x15% of the average particle size. The majority here means 70% of the particles or more, preferably 80% of the particles or more and most preferably 90% of the particles or more.
In a further preferred embodiment of the present invention, blends of 2 of 3 particles of different sizes are used. The resulting bi- or tri-modal particle size distribution has the advantage that the amount of space in the insulating body occupied by the particles can be increased in comparison to a filling which uses only particles of one size. For instance, if the insulating body is filled with spherical particles, the maximum volume occupied by these spheres is approximately 60%, the rest being void volume. With a mixture of spheres containing larger spheres and smaller spheres, whereby the diameter of the smaller spheres corresponds to approximately 40% of the diameter of the larger spheres, the 60% volume coverage achieved with the larger spheres can be increased by approximately 7%. In this context, it may also be particular advantageous to use mixtures of particles with various shapes. For instance, if a mixture of larger spheres and cubes with an edge length that corresponds to 40% of the diameter of the larger spheres is used, a volume coverage of more than 70% can be achieved. In this context, cylindrical shapes and the like may also add to a high volume coverage of the particulate materials.
The insulating compound used in the present invention is a cross-linkable polymeric material. Prior to the crosslinking, the insulating compound needs to be liquid enough to allow for easy filling of the insulator body. Accordingly, its viscosity in the non-cross-linked state should preferably not exceed 2000 mPas (Brookfield) at 23xc2x0 C., preferably its viscosity is below 1500 mPas, and most preferably its viscosity is in the range of 700 to 1000 mPas.
The density of the insulating compound is not subject to any particular limitation. However, usually the density is in the range of 0.95 to 1.1 g/cm3 in the non-cross-linked state.
The present invention contemplates that the insulating compound is filled into the insulator body in addition to the above particulate material. Once the insulating compound is filled into the insulator body, it undergoes a cross-linking reaction, so as to form a soft matrix surrounding the particulate material and the cable. Accordingly, the insulating compound is a cross-linkable compound, which upon cross-linking should exhibit the necessary electric properties and be of a soft, gel-like consistency. The softness of the cross-linked insulating compound is desirable, as it allows for the compensation of mechanical stress on the insulating filling.
Typically, the insulating compound upon cross-linking and in the absence of the particulate material has a hardness according to DIN ISO 2137 of 200 to 500 mm/10, preferably 250 to 400, and most preferably 290 to 350 mm/10. Very good results have been obtained with insulating compounds which exhibit a hardness upon cross-linking of 310 to 350 mm/10.
After the cross linking the viscosity of the insulating compound under all operating conditions, e.g. from xe2x80x9440xc2x0 to 100xc2x0 C., is such that in can be permanently contained in the insulator body without necessitating gas- or liquid-tight seals. In other words, the cross linked insulating compound forms a soft but solid body. It moreover, at least in admixture with the particulate material has no apparent and measurable viscosity.
In order to further reduce mechanical stress, it is also desirable at the thermal conductivity at 20 to 150xc2x0 C. according to DIN 52612 is in the range of 0.15 W/mK to 0.3 W/mK, particulary preferred are thermal conductivities around 0.2 W/mK.
For the same reason it is also preferred that the coefficient of linear expansion of the insulating compound in the crosslinked state is small, i.e. in the range of 200xc3x9710xe2x88x926 m/mK to 400xc3x9710xe2x88x926 m/mK, preferably between 300xc3x9710xe2x88x926 m/mK and 350 m/mK.
In terms of the electrical properties, the dielectric strength (1 mm sheet, IEC 243-2) should be in the range of 18 to 30 kV/mm, whereby values between 20 and 25 KV/mm and in particular values around 23xc2x110% KV/mm are preferred.
The volume resistivity measured at 23xc2x0 C. according to DIN IEC 93 should preferably be in the range of 5xc3x971015 to 5xc3x9710xe2x88x9216 xcexa9cm. Very good results have been obtained with insulating compounds having a volume resistivity of 1016xc2x110% xcexa9.
It is also preferably that the relative permittivity of the insulating compound upon cross linking (VDE 030 T4, 50 Hz) should be between 2.5 to 3, preferably between 2.7 and 2.9.
Insulating compounds of the above type can be of diverse chemical structure. The common demoninator, of course, is that they should be capable of being cured in the insulator body and satisfy the above requirements particularly in regard to the softness. The curing may thereby be effected according to various methods known in the art. However, addition curing processes that proceed at ambient temperature are preferred.
Preferred insulating compounds are modified hydrocarbons, such as polybutadiene modified polyolefins and silicone polymers.
Preferred silicone-based insulating compounds can be derived from a first silicone component with the following chemical structure: 
wherein R1, R2, R3, R4, R5 and R6 independently represents alkyl groups with 1 to 5, preferably 1 to 3 carbon atoms or phenyl groups, m and n represent integers other than zero and the ratio of m to n is in the range of 0.01 to 1 preferably in the range of 0.05 to 0.5 and most preferably in the range of 0.1 to 0.5.
Compounds of this type can be cross-linked with a second silicone based component of the following formula: 
wherein R1, R2, R3 and R4 are defined as above and wherein p represents an integer that is different from zero, preferably an integer of 1 to 50, more preferably an integer of 1 to 20 and most preferably an integer of 1 to 15.
The ratio of the silicone hydride compound to the vinyl group containing silicone is usually in the range of 0.3 to 1 and preferably between 0.5 and 0.8. Accordingly, the mechanical properties of the insulating compound can best be adjusted by choosing a suitable number of vinyl groups in the above vinyl group containing silicone polymer.
The cross-linking of these two components can be effected by methods known in the art. A preferable method to effect the hydrosilitation is to use noble metal catalysts and in particular platinum catalysts, such as metallic platinum, which may be used on a support, platinum salts and chloro platinum acid as well as respective complexes.
The amount of catalyst to be used for the hydrosililation cross-linking reaction is not particularly limited. However, usually 5 to 100 mg noble metal catalyst per kg vinyl component is advisable. The amount of platinum catalyst should reflect a compromise between the costs for the noble metal catalyst and the desired curing time.
In one embodiment of the present invention, the insulating compound should be chosen as such that it adheres both the particulate material and the interior walls of the insulator body. This may either be effected by chemical bonds, which may be formed in the course of the cross-linking reaction or by physical forces, namely by choosing a material with a particular tackiness.
In another embodiment, the particles are electrically conducting, for example made from conductive polyethylene. The solid particles may also be provided with a thin film to reduce the surface tension. The thin film can be a thin layer of a silicone oil. It is also preferred that the solid particles are made of a low smoke toxic-free material.
It should be noted that the present disclosure only reflects what the inventors presently conceive as the best mode of the invention and that further variations and modifications of the invention may be carried out on the basis of the teachings contained herein. In particular, the invention comprises embodiments consisting of combinations of features which have been separately described in the description and/or in the claims.